Connection between a rod and crosshead in a reciprocating compressor

ABSTRACT

A connection is disclosed between a rod and crosshead in a reciprocating compressor comprise threaded units, an anti-rotational securing device and axial positioning elements, wherein the threaded units consist of a first inner lock nut, provided with an inner thread onto which there is screwed a threaded end portion of the rod and provided with two outer threads with a different diameter, one of which is for connection to the left-handed anti-rotational securing device, and the units also consist of a second outer lock nut in the inner diameter of which a thread is provided.

BACKGROUND AND SUMMARY OF THE INVENTION

[0001] The present invention relates to means for connection between arod and crosshead in a reciprocating compressor.

[0002] Priority is claimed to Italian patent application MI2001/A00/1757, filed Aug. 9, 2001.

[0003] The most common type of reciprocating compressors consist of oneor a plurality of cylinders inside which pistons provided withreciprocating motion slide.

[0004] As far as the kinematic control mechanism for the reciprocatingmotion of the piston is concerned, reference is made by way of exampleto the kinematic mechanisms which consist of a connecting rod and crankor a connecting rod, crank and crosshead.

[0005] The second type of transfer of reciprocating motion proposed isapplied to machines which do not have a direct connection between thefoot of the connecting rod and the piston, but require interposition ofa stem or rod.

[0006] However, the rod is not designed to withstand transverse forcesand it is therefore necessary to provide at its articulation with thefoot of the piston a straight guide which forces the center of thearticulation to move along the axis of the cylinder, constituted by theso-called crosshead.

[0007] The crosshead consists of a body provided with a journal or pinfor articulation with the connecting rod, and with runners, wherein theaxis of the pin is at right-angles to the axis of the rod and of therunners, i.e. it is disposed in the shape of a cross relative to thelatter.

[0008] In general the runners are produced with a cylindrical surfacemade of cast iron or steel or light alloy, are integral with the body oradded onto the latter and are covered with anti-friction metal. Therunners slide in guides with a complementary cylindrical surface.

[0009] In general the body is produced from cast iron, cast steel orforged steel.

[0010] Hereinafter for the crosshead reference will be made by way ofexample to the configuration with a pin and two runners, which isdesigned for connection to drive shafts which can rotate in bothdirections, thus exchanging transverse forces with both the guides.

[0011] At present the means for connection between the piston rod andthe crosshead in a reciprocating compressor consist of common threadedunits, i.e. they consist of a nut and a single lock nut.

[0012] For correct fitting use is also made of elements to stoprotation, of the key type.

[0013] In the fitting solutions generally used the means for connectionbetween the rod and the crosshead, i.e. the nut and lock nut, abutsupports, for example flanges, which are finally connected to thecrosshead by means of tie rods.

[0014] The tightening of the threaded units, such as to apply apredetermined load which assures the hold of these means for connectionin use can be carried out manually for example, in the case ofreciprocating compressors with low performance, or hydraulically.

[0015] The hydraulic tightening in the case of high-performance machineswhich must guarantee maximum reliability and safety makes it possible toobtain better preloading and thus better hold of the connection betweenthe rod and the crosshead.

[0016] The hydraulic tightening of the means for connection between therod and crosshead in reciprocating compressors is carried out forexample by positioning the hydraulic jack on the rod, in the sectionbetween the nut and the lock nut, such as to abut between the nut andthe support, which in turn is secured to the crosshead.

[0017] This solution is structurally more complicated, sincecorresponding holding means must be provided for the correct functioningof the hydraulic jack.

[0018] The main disadvantage of the means for connection between the rodand crosshead in a reciprocating compressor such as those used atpresent relates to limitation of the load applicable to the rod duringfunctioning. On the basis of the foregoing information this value isdependent on the nominal value of the preloading applied during fittingand above all on the extent of the relaxation once the tightening hasbeen completed, which is expressed in the form of a relaxationcoefficient.

[0019] In fact, in the present fitting solutions, in a reciprocatingcompressor the means for connection between the rod and the crosshead,i.e. the nut or lock nut, are not subjected to stress during the fittingstep but only at the moment of release of the traction load applied inorder to connect the rod to the crosshead. The coefficient of relaxationis therefore not optimized with these fitting solutions.

[0020] In reciprocating compressors the means for connection between therod and crosshead constitute a critical characteristic for thereliability and performance of the machine.

[0021] The dynamic stresses induced by the cyclical load, which ischaracteristic of the functioning of reciprocating compressors, canlimit the performance of these means for connection above all from thepoint of view of fatigue strength, which is characteristic of thereciprocating motion functioning system.

[0022] The means for connection between the rod and crosshead of areciprocating compressor used hitherto do not however constitute anoptimized response in terms of performance of the machine.

[0023] A means is disclosed for connection between the rod and crossheadin a reciprocating compressor which eliminate the above-describeddisadvantages.

[0024] A means is disclosed for connection between the rod and crossheadin a reciprocating compressor which make it possible to obtain higherperformance levels in terms of loads which can be applied to the rod forthe same nominal preloading and type of thread of the nuts.

[0025] A means is disclosed for connection between the rod and crossheadin a reciprocating compressor which are particularly simple andfunctional, with low costs.

SUMMARY OF DRAWINGS

[0026] The characteristics and advantages of means for connectionbetween the rod and crosshead in a reciprocating compressor according tothe present invention will become more apparent from the followingdescription, provided by way of non-limiting example with reference tothe attached schematic drawings, in which:

[0027]FIG. 1 is a lateral elevated view partially in cross-section of acrosshead of a reciprocating compressor on which a rod is fitted bymeans for connection, which are the subject of the present invention;

[0028]FIG. 2 is a lateral view of FIG. 1, which shows a crossheadbearing a rod and means for connection, which are the subject of thepresent invention, partially in cross-section on different planes;

[0029]FIG. 3 is a plan view partially in cross-section of FIG. 1; and

[0030]FIG. 4 shows an enlarged detail of the means for connectionbetween the rod and crosshead in a reciprocating compressor, which isthe subject of the present invention.

DETAILED DESCRIPTION OF DRAWINGS

[0031] The figures show means for connection between the rod 12 andcrosshead 13 in a reciprocating compressor, indicated as 10 as a whole,consisting substantially of threaded units, such as two lock nuts, ananti-rotational securing device and axial positioning elements.

[0032] The crosshead 13 shown in FIGS. 1 to 3 consists of a body 14 andrunners 15 with a cylindrical surface which slide in complementaryguides 16. As shown in FIG. 2 the body of the crosshead 14 has asubstantially hollow annular shape and is provided with a seat 17 toaccommodate a pin 18, disposed with its axis 19 at right-angles to thedirection of sliding X of the runners 15.

[0033] The pin 18, which is provided with countersunk threaded holes 20,constitutes the point of articulation with the connecting rod, not shownin the figures.

[0034] In the section in which it engages with the crosshead 13 the rod12 has a countersunk threaded hole 21, which is provided in a circularhead surface 22 along an axis 23 of the rod, as shown in detail in FIG.4.

[0035] The rod 12 has a threaded end portion 24, a tapered section 25, acentering portion with a larger diameter 26 and finally, correspondinglyconnected to the latter, a stop surface 27 which is inclined by an anglea relative to the axis 23 of the rod.

[0036] A conical ring 28 forms part of the axial positioning elements ofthe assembly constituted by the rod and piston relative to the fixedparts of the cylinder.

[0037] The positioning ring 28 has a frusto-conical mouth 29 which iscomplementary to the inclined stop surface 27 of the rod.

[0038] A flange 30 constitutes a further axial positioning element andconsists of a sleeve 31 to contain the rod and lock nuts, which extendsfrom a connection plate 32, provided with four holes 33 disposed inpairs on opposite sides.

[0039] In the holes 33 there are engaged an equivalent number of tierods 34, which are screwed onto the pin 18 accommodated in the body ofthe crosshead 14, in order to connect the flange 30 to the crosshead 13.

[0040] The portion with a sleeve 31 of the flange 30 has an innerdiameter which is equal to the maximum diameter of the centering portion26 of the rod 12.

[0041] At one end of the flange 30, on the inner diameter, there areprovided cylindrical seats 35, 36 against which there abut respectivelya first inner lock nut 37 and a second outer lock nut 38.

[0042] The first inner lock nut 37 consists of a cylindrical elementwith an elongate shape, provided with an inner thread 39 which extendsalong its entire length. Externally there are present three separatesections with an increasing diameter. A first cylindrical section with asmaller diameter 40 is followed by a first outer thread 41 with a largerdiameter and finally a left-handed outer thread 42 with a maximumdiameter.

[0043] A lateral annular surface 43 of the first lock nut 37 is alsoprovided with one or more holes 44 which co-operate with theanti-rotational securing element for the rod.

[0044] On the inner surface of the second outer lock nut 38 there ispresent a thread 45, and on the other hand the outer surface has at oneend a cylindrical section 46, in which there is provided a hole 47 forthe intervention of the tightening means and a cylindrical section witha smaller diameter 48.

[0045] The anti-rotational securing device consists of a ring nut 49 tosecure the lock nuts 37 and 38, which is provided with a hole 50 for theintervention of the tightening means, and in the inner diameter of whichthere is provided a left-handed thread 51. The ring nut 49 is providedwith a thin end lip 52, which, when the rod 12 is fitted, is positionedin the vicinity of a seat 53 provided in the flange 30, as shown in adetail in cross-section in FIG. 3.

[0046] The anti-rotational securing device also comprises an element 54to secure the rod which has a cylindrical end portion provided with aleft-handed thread 55, a hole 56 for the intervention of the tighteningmeans and an outer limb 57. This outer limb 57 is provided in the formof a circular ring consisting of a thinned wall, or alternatively it canbe produced for example in the form of tabs disposed annularly.

[0047] The anti-rotational securing device must assure a stableconnection in conditions of use, by preventing the connected elementsfrom becoming unscrewed from one another.

[0048] The means for connection 10 between the rod 12 and the crosshead13, which is the subject of the present invention, assure above allstable connection of the rod 12 to the flange 30, by means of the use ofthe double lock nut 37 and 38. These means thus assure connection of theflange 30 which is integral with the rod 12, to the body of thecrosshead 14.

[0049] Since the rod is connected to the piston, which is already fittedinside the cylinder, the conical positioning ring 28 is inserted on thefree ends of the rod 12 until it abuts the corresponding inclinedsurface 27, as can be seen from the enlarged detail in FIG. 4.

[0050] The conical ring 28, which is worked to the exact size during theinitial fitting, serves the purpose of carrying out correct axialpositioning in relation to the fixed parts of the cylinder of the unitconstituted by the rod and piston.

[0051] The flange 30 is thus inserted on the rod 12 until it abuts theconical ring 28, and also co-operates in carrying out the centering onthe diameter of the rod.

[0052] The second outer lock nut 38 is screwed entirely onto the outerthread 41 with a smaller diameter of the first inner lock nut 37. Theinner thread 39 of this first lock nut 37 then engages with that of theend portion 24 of the rod 12.

[0053] In this step the cylindrical sections with a smaller diameter 40and 48 of the lock nuts 37 and 38 are inserted at least partially in therespective cylindrical seats 35 and 36 in the flange 30. In thiscondition only the first inner lock nut 37 abuts the flange 30. On theother hand the second outer lock nut 38 will be made to abut the flange30, within its circular seat 36, in the successive fitting steps.

[0054] In order to tighten the means for connection between the rod 12and the crosshead 13, the hydraulic jack is screwed onto the freesection of the inner thread 39 of the first lock nut 37.

[0055] Pumping the oil to the required pressure provides the correctpretensioning of the end 24 of the rod 12 and deformation of the threads39 of the first lock nut 37 engaged on the rod.

[0056] Whilst keeping the oil pressure constant, the tightening of thesecond outer lock nut 38 is completed until the lock nut abuts the seat36 provided in the flange 30.

[0057] When the pressure is released in order to remove the hydraulicjack from the lock nut 37 there is reduced relaxation of the nominalpreloading owing to the opposing thrust action exerted by the secondlock nut 38.

[0058] The extent of relaxation of the preloading is commonly expressedin the form of coefficient of relaxation.

[0059] Finally, the application of the elements which constitute theanti-rotational securing device guarantees the maximum reliability inconditions of use, thus preventing loosening of the connection betweenthe lock nuts.

[0060] The securing ring nut 49 is screwed fully onto the outerleft-handed thread 42 of the first lock nut 37. On completion of theoperation the end lip 52 is folded inside the corresponding seat 53provided in the flange 30, as shown in FIG. 3, such as to preventrelative rotation between the ring nut 49 and the lock nut 37 and thusto prevent the latter from becoming unscrewed or loosened in use.

[0061] The securing ring nut 49 thus carries out the function ofpreventing possible loosening of the second lock nut 38 and thus also ofthe inner lock nut 37 which is integral with the latter, thereforerendering the lock nuts integral with the flange 30.

[0062] Finally, the securing element for the rod 54 prevents relativerotation between the rod 12 and the lock nut unit 37 and 38 unitrelative to the flange 30, as previously described.

[0063] The securing element for the rod 54 is screwed onto theleft-handed thread of the countersunk hole 21 provided in the circularhead surface 22 of the rod 12 itself.

[0064] The outer limb 57 with a small thickness is thus folded into atleast one hole 44 present in the lateral annular surface 43 of the firstlock nut 37.

[0065] As prescribed by the standards API618, the anti-rotationalsecuring between the rod 12 and flange 30, and consequently between therod 12 and the crosshead body 14, is produced without using deviceswhich are engaged in the highly preloaded section of the rod, such ascentering keys. Since the rod is fatigue stressed, in fact theanti-rotational securing devices used hitherto jeopardize the intactnessof the latter.

[0066] The flange 30 is connected to the body of the crosshead 14 bymeans of fitting operations which are known to persons skilled in theart, and are therefore listed only briefly.

[0067] The flange 30 is connected to the body of the crosshead 14 bymeans of four tie rods 34, which are screwed into the countersunk holes20 in the pin 18 for articulation of the foot of the connecting rod,which pin is inserted in the seat 17 provided in the body of thecrosshead 14.

[0068] The four tie rods 34 are tightened by means of hydraulic jacks,simultaneously for each opposite pair of tie rods in the cross.

[0069] The connection according to the present invention is also usedfor crossheads other than the type previously described.

[0070] In use a plurality of devices therefore contribute towardsassuring the hold of the threaded connections between the rod and theflange. As well as having anti-rotational securing elements which areprovided with a left-handed thread, these securing elements are alsoprovided with thin limbs, which are folded inside holes in adjacentelements.

[0071] The means for connection between the rod and crosshead of areciprocating compressor, which is the subject of the present invention,have the advantage of minimizing the decrease in preloading owing to theresilience of the parts concerned.

[0072] By use of the means for connection which are the subject of thepresent invention, the common relaxation values, which in general areequal to 40-50% of the nominal preloading, are reduced to relaxation of20-25% of the nominal load.

[0073] The use of the means for connection between the rod and crossheadof reciprocating compressors, which is the subject of the presentinvention, thus makes it possible to obtain higher performance levels interms of loads which can be applied to the rod for the same nominalpreloading and type of thread of the nuts.

[0074] During the fitting step the hydraulic jack advantageously acts onthe inner thread of the inner lock nut, the threads of which areimmediately deformed by the action exerted on them, and continue to bedeformed even after the tightening pressure has been released, owing tothe action of the second lock nut.

[0075] In addition, the anti-rotational securing element of the rod,which assures the hold of the connection between the rod and flange, andconsequently between the rod and the body of the crosshead, as requiredby the standards API618 fourth edition, has considerable advantages fromthe point of view of mechanical strength.

[0076] In fact, elimination of the common anti-rotational securingdevices, for example of the key type, which engage in a highly preloadedsection of the rod, makes it possible to increase advantageously thefatigue strength of the rod.

[0077] Numerous modifications and variations, all of which come withinthe scope of the invention, can be applied to the means for connectionthus designed, between the rod and crosshead of a reciprocatingcompressor, which is the subject of the present invention; furthermoreall the details can be replaced by technically equivalent elements. Inpractice any materials and dimensions can be used, according to thetechnical requirements.

What is claimed is:
 1. A connection between a rod and a crosshead in areciprocating compressor comprising: threaded units, an anti-rotationalsecuring device and axial positioning elements, wherein the saidthreaded units consist of a first inner lock nut and a second outer locknut in the inner diameter of which a thread is provided, wherein thesaid first inner lock nut is provided with an inner thread onto whichthere is screwed a threaded end portion of the said rod and is providedwith two outer threads with a different diameter, wherein one of thesaid two outer threads for connection to the said anti-rotationalsecuring device is left-handed.
 2. A connection as in claim 1, whereinsaid anti-rotational securing device comprises a securing ring nut forthe said lock nuts, in the inner diameter of which there is provided aleft-handed thread, and of an element to secure the rod, which has acylindrical end portion provided with a left-handed thread forengagement with a countersunk threaded hole, wherein the said hole isprovided in a circular head surface of the said rod.
 3. A connection asin claim 2, wherein said securing nut includes an end lip foldableinside a seat provided in one of said axial positioning elements.
 4. Afor connection as in claim 2, wherein said element for securing the rodincludes an outer limb foldable into at least one aperture in a lateralannular surface of the said first lock nut.
 5. A connection as in claim1, wherein said axial positioning elements comprise a positioning ringfor said rod relative to fixed parts of a cylinder and a flange forconnection to the said crosshead.
 6. A connection as in claim 5, whereinsaid connection flange comprises a sleeve to contain the said rod andthe said lock nuts, as well as of a plate for connection to thecrosshead.
 7. A connection as in claim 5, wherein said positioning ringhas a frusto-conical mouth surface which is complementary to a stopsurface of the said rod, and is inclined by an angle (a) relative to anaxis of the said rod.
 8. A connection as in claim 6, wherein saidcontainer ring has an inner diameter which is equal to the maximumdiameter of a centering portion of the said rod.
 9. A connection as inclaim 5, wherein said first inner lock nut and the said second outerlock nut each have an end cylindrical section having a smaller diameterfor engagement in cylindrical seats provided in the inner diameter ofthe said flange.